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SUMMARY:Cellulose: the plant kingdom's strong material - Dr Michael Jarvis
 \, Glasgow University
DTSTART:20150224T140000Z
DTEND:20150224T150000Z
UID:TALK57715@talks.cam.ac.uk
CONTACT:Hannah Ambler
DESCRIPTION:There is much interest in protein-based strong materials from 
 animals as inspiration for the design of new synthetic polymers and compos
 ites. Cellulose is equally inspirational\, but different. Microfibrils of 
 pure crystalline cellulose\, which are a few nanometers thick\, have tensi
 le properties comparable with Kevlar and a strength: weight ratio better t
 han steel. The tensile stiffness of crystalline cellulose is based on a re
 markable form of co-operation between hydrogen bonds and the parallel cova
 lent bonding that links the glucose units within each polymer chain. The m
 echanism of this co-operative elastic stiffness was elucidated by a combin
 ation of crystallography under tension and vibrational bandshift analysis.
  The underlying principle may be termed 'molecular leverage'\, a concept w
 ith scope for application to other biopolymers.\nIn plant materials like w
 ood\, cotton and the walls of growing plant cells\, cellulose is not found
  in pure crystalline form. Its hydrogen-bonding pattern is partially disor
 dered\, particularly at the surfaces of the microfibrils. It is associated
  with other polysaccharides\, loosely termed hemicelluloses\, that have si
 milar structures but less capacity for intramolecular hydrogen bonding. Co
 mpared to cellulose\, hemicellulose chains associate less with one another
  and more with water. In materials like wood there is a disorder gradient 
 from the crystalline centres of the microfibrils\, through their surfaces 
 and bound hemicelluloses\, to the viscous\, hydrated matrix between. When 
 such materials are under tension they extend by a combination of elastic s
 tretching of cellulose and shear between the cellulose microfibrils\, in a
  ratio that depends on the cellulose orientation. How the shear component 
 dissipates energy to retard fracture is not well understood\, although it 
 clearly differs from energy dissipation in strong animal materials like sp
 ider silk. Stress-relaxation experiments will be described that make use o
 f vapour-phase deuteration\, combined with FTIR bandshift analysis and neu
 tron diffraction\, to distinguish crystalline cellulose from the hydrated\
 , less ordered\, domains. \nCellulose orientation is under tight developme
 ntal and genetic control: it is the principal factor that underlies both p
 lant morphogenesis and the mechanical properties of plants. Understanding 
 the mechanical function of cellulosic nanostructures in vivo will give us 
 new insights into the design of new\, strong composites and the performanc
 e of traditional\, sustaina\n
LOCATION:Department of Biochemistry\, Hopkins Building\, Seminar Room 1
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